April 5, 2012

Tracker Mini Update - Accuracy Testing

Surprisingly, and in voilation of the weather forecast, skies are clear over North Auckland on Easter Eve (is that a thing?) Which reminded me that the epoxy on my tracker cog-and-nut assembly must be set. Time to test!

Before reassembing the barn door, I decided a bit more scientific testing was in order. First I threaded the cog onto a bolt and attached it to my hand drill. Revving it up, I could get a feel for how much wobble there was as it revolved - was the nut centered on the cog now? Looked pretty good! It became clear there was a reasonable amount of play in it at right angles to rotation axis, but that wasn't too concerning as the wobble was parallel to the bolt, once assembled the other componants would mitigate this.

I was about to head outside, when a thought from earlier in the week resurfaced. Why not video the mechanism turning, and do some metrics on the resulting video to see how smoothly the thing was running? Maybe the motor itself wasn't that stable for eg? This would give me a fairly accurate sense of things without needing to touch the SLR. My cell-phone camera was drafted in, 30fps providing plenty of "resolution" time-wise.

Below is the resulting graph, generated by stepping through with quicktime and recording in a spreadsheet the elapsed frames as each tooth of the cog passed a set point. All in all, a pretty good result! The graph looks nice and linear, within the margin of error of the fairly fuzzy video I was working with. Cogs across the bottom axis, frames up the y-axis.